This invention relates generally to printers and, more specifically, to printing techniques for color printers.
Thermal inkjet printers operate by selectively energizing heating elements to expel droplets of ink through associated nozzles. Each heating element, which is typically a pad of resistive material about 50 .mu.m by 50 .mu.m in size, is located in a chamber filled with ink supplied from an ink reservoir. A nozzle plate, defining an array of nozzles, overlies the various chambers. Upon energizing a particular heating element, a portion of the ink in the chamber is vaporized and a droplet of ink is expelled through an associated nozzle toward the print medium, whether paper, fabric, or the like. The firing of the heating elements is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the heating elements. The arrangement of printed dots may form any pattern, such as text or graphics.
One embodiment of an inkjet pen is described in Hewlett-Packard's U.S. Pat. No. 5,278,584, by Brian J. Keefe, et al., entitled INK DELIVERY SYSTEM FOR AN INKJET PRINTHEAD, incorporated herein by reference.
Some of Hewlett-Packard's color printers will include a scanning carriage housing one inkjet pen containing black ink, as well as housing one or more color inkjet pens for printing cyan, magenta, or yellow ink. These color inkjet pens typically have a resolution of 300 dots per inch (dpi) in both the x and y axes and a printhead width (along the direction of paper travel) on the order of one-third inch.
During printing, the pens are scanned across the width of the medium to be printed upon while the medium remains stationary. During each scan, the cyan, magenta, and yellow inks are ejected by the nozzles under control of the microprocessor to create the desired colors and patterns. Each completed scan across the medium can print a swath approximately as wide as the width of the entire nozzle array of a pen. After each scan, during which all colors and the black ink may be printed, the medium is moved forward the width of a swath, and the pens begin printing the next swath.
The color pens may be either a single tricolor pen, having three sets of nozzles for cyan ink, magenta ink, and yellow ink, respectively, or consist of three separate and identical pens each containing a respective one of the cyan, magenta, or yellow inks. Secondary colors are obtained by overlapping the various colors of ink or printing different color dots adjacent one another.
Various problems arise when printing using the above technique. Because of the wet ink being drawn into the surrounding dry media by capillary action, the edges of the printed images tend to become less defined. Also, when inks of differing colors are printed adjacent to each other, the different color inks tend to bleed into each other. Another problem encountered in inkjet printing is paper cockle. The ink used in inkjet printing is typically of a liquid base. When the liquid ink is deposited on wood-based papers, it absorbs into the cellulose fibers and causes the fibers to swell. As the cellulose fibers swell, they generate localized expansion, which, in turn, causes the paper to warp (cockle) in these regions. This can cause a degradation of print quality due to uncontrolled pen-to-paper spacing, and can also cause the printed output to have a low quality appearance due to the wrinkled paper.
What is desirable is a method for controlling the color pen(s) in a scanning carriage to mitigate the above-mentioned problems of ink bleed and cockle.